1. Field of the Invention
The invention relates to a process for introducing gases into liquids, in particular for contacting gaseous and liquid reactants, and to the use of an apparatus for carrying out this process.
2. Description of the Prior Art
In a number of processes, the problem arises of introducing gases into a liquid, either in order to disperse a gas in a liquid or to dissolve it, or in order to effect a chemical reaction between a gaseous reactant and a liquid reactant.
If a gas, say, is then passed through the liquid, the gas is quantitatively absorbed by the liquid only in the case of a high reaction rate and a small mass transfer resistance. In all other cases, only a more or less small proportion of the gas is absorbed by the liquid. The residual part of the gas remains unutilized.
This is unsatisfactory because the gases used frequently just be prepared at considerable cost.
If, for instance, toxic gases, such as hydrogen cyanide or carbon monoxide are used, an expensive destruction or reprocessing is indispensable. Difficulties arise also in the cases where the liquid tends to foam under the action of gases.
When air, oxygen or ozone are used for the oxidation of organic substances, the formation of highly explosive exit gas mixtures must be expected. An example of this is the oxidative coupling reaction of 2,6-disubstituted phenols. In such cases, a considerable expenditure on safety engineering is necessary in order to ensure that the reaction can proceed without hazard. Thus, for example, air saturated with toluene is explosive in the range from about 4.degree. to 42.degree. C.
Numerous types of apparatus are known from the state of the art, which make intimate contact of a gas with a liquid possible, for example in Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopaedia of Industrial Chemistry], 4th edition, volume 1, pages 225 et seq., (in particular pages 226 and 227), volume 2, pages 275 et seq., and volume 3, pages 357 et seq. (in particular page 359). These are plate columns and packed columns, spray devices, nozzles and stirred kettles which, in addition to different types of agitator, can contain various further inserts. However, the processes working with such types of equipment fundamentally suffer from the disadvantages described.
European Published specification No. 0,087,670 describes a process for avoiding an explosive gas phase in a vertical gas/liquid reactor with an enveloping tube, closed at the top, and nozzles, through which liquid jets are intended to emerge. This process appears to be involved, since the desired gas can be introduced into the reactor only after inert gas has been admitted first and has been so completely dispersed in the liquid that a relatively large coherent gas space is not longer present.
This process has two further disadvantages:
1. The measure of forcing a liquid through nozzles and thus achieving thorough mixing cannot be satisfactorily carried out if the liquid drawn in contains considerable quantities of dissolved gas constituents.
2. As is known, the dispersion of gases by means of jet nozzles requires a high energy consumption.
There are comparatively few processes which utilise horizontally disposed reactors.
In a number of scientific investigations (Ando et al., Journal of Chemical Engineering of Japan 5, 193 (1972); Aldo et al., Int. Chemical Engineering 11, 735 (1971) and Ando et al., AlChE 27, 599 (1981)), the influence of individual apparatus characteristics on the gas absorption and the stirrer power consumption in horizontally disposed reactors was investigated. In these experiments, the fraction of the gaseous phase always remains markedly above 30% of the reactor volume and the reaction mixture is thus inhomogeneous. This manifests itself in a non-uniform power consumption by the stirrer.